Astrocytes: what functions do these glial cells perform?
Glial cells are fundamental to the functioning of the nervous system as they provide structure, nutrients and protection to the neurons, in addition to carrying out other relevant tasks.
In this article we will talk about astrocytes, one of the most common types of glia . We will describe their morphology and their main functions and we will differentiate the three types of astrocytes that have been identified.
What are astrocytes?
Astrocytes are a type of glial cell located in the central nervous system , that is, in the brain and spinal cord. Like the rest of the glial system, astrocytes play supporting roles in relation to neurons, the main cells of the nervous system from a functional point of view.
These glial cells have a shape slightly reminiscent of a star ; their name is derived from this fact, since the Greek and Latin words “astron” and “astrum” are translated as “star” or “celestial body”. Such a structure is due to the fact that they have many extensions (“feet”) that connect the soma with other nearby cells.
Astrocytes are formed from cells of the ectoderm , the layer of the embryonic disc from which the nervous system and the epidermis arise, during the body’s early development. Like most glia, astrocytes start from undifferentiated cells similar to those that give rise to neurons.
Glial cells or glia
As we know, neurons specialize in the transmission of nerve impulses. They are therefore very effective in this task, but they need the support of other types of cell in order for the nervous system to function properly; this is where the glia or neuroglia, that is, all the glial cells, which make up 50% of the nerve mass, come in.
The specific roles of these cells depend on the type of glia referred to. In general, we can say that they serve mainly to give physical and structural support to neurons , to isolate them from each other, to provide them with nutrients and oxygen and to eliminate waste products and pathogens.
Other particularly relevant glial cells are the microglia, which perform defensive and immunological functions in the brain and spinal cord, oligodendrocytes and Schwann cells , which form the myelin sheaths that surround the axons and accelerate neuronal transmission in the central and peripheral nervous system, respectively.
For a long time it was believed that the function of astrocytes was basically structural: to “fill in the gaps” left by neurons in the nervous system.
However, research in recent decades has shown that their role, like that of other glial cells, is much more complex.
1. Nerve structure
Astrocytes and glia in general play an important role in providing physical support to neurons , so that they stay in place, as well as regulating the transmission of electrical impulses. Astrocytes are the most abundant glia in the brain, so their structural role is especially important in this organ.
2. Blood-brain barrier
These glial cells act as intermediaries between the neurons and the circulatory system , specifically the blood vessels. In this sense they fulfil a filtering function, so that they constitute a part of the blood-brain barrier, formed by closely linked endothelial cells of the brain.
3. Nutrient supply
The connection of the astrocytes to the vascular system allows them to obtain nutrients, such as glucose or lactic acid, from the blood and to provide them to the neurons.
4. Fagocitation and waste disposal
Similarly, astrocytes collect waste products from neurons and transport them into the blood so that they can be eliminated . In addition, when an injury to the nervous system occurs, astrocytes move towards it to phagocytize or eliminate the dead neurons, forming scars in the damaged area by accumulating in it.
5. Glycogen reserve
Astroglia may also have the function of storing glycogen, which serves as an energy reservoir, so that neurons can access these reserves in times of need.
6. Extracellular space regulation
Astrocytes help to maintain the ion balance in the extracellular space; in particular, they reverse the excessive accumulation of potassium because they are very permeable to these molecules.
There are three types of astrocytes that are differentiated by the cell lineage from which they come, that is, the type of neuroepithelial cells from which they originate. Thus, we can distinguish between fibrous, protoplasmic and radial astrocytes .
These astrocytes are located in the white substance of the nervous system, that is, in the areas formed predominantly by myelinated axons. They are characterized by their low number of organelles (cellular subunits with differentiated functions).
Protoplasmics contain many organelles and are the most numerous type of astrocyte . They are located mainly in the grey matter of the brain, which is composed mainly of cell bodies.
Radial glia plays a decisive role during the process of cell migration, since neurons “travel” through the nervous system relying on this type of astrocyte. However, radial glial cells are also active in adulthood, such as Bergmann’s cells located in the cerebellum.
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